1. Field the Invention
The present invention relates to a misfire deciding method and system for an internal combustion engine.
2. Description of the Related Art
As a misfire deciding method in an internal combustion engine, a method has been proposed for directly measuring the pressure in a combustion chamber, for example, by forming a pressure conduit leading to the combustion chamber in a cylinder head and by arranging a partition type pressure sensor in the pressure conduit. The formation of this pressure conduit in the cylinder head, however, involves complex machining which inevitably raises the manufacturing cost. A more convenient method for measuring the internal pressure uses a pressure sensor (hereinafter called a xe2x80x9cgasket type pressure sensorxe2x80x9d) which is mounted on the mounting seat of a spark plug, as disclosed, for example, in Japanese Patent Laid-Open No. 290853/1994.
FIG. 3 exemplifies an internal pressure profile as measured by an internal pressure sensor for one cycle in the combustion engine. The solid curve indicates the profile for normal combustion timing, and the single-dotted curve indicates the profile for a misfire timing. When an intake valve is closed, the inside of the combustion chamber is sealed, and the mixture is compressed as a piston rises, so that the internal pressure rises. Moreover, the spark plug sparks at a crank angle (or an ignition timing) before top dead center (TDC). When the mixture is normally ignited with that spark, the internal pressure is further raised by the explosion of the mixture. By this pressure rise, the rise in the internal pressure is continued for a while, even after the piston passes through the TDC and turns downward, and the internal pressure is reduced after the piston goes down to some extent. Therefore, the internal pressure measured profile assumes an asymmetric shape in which the peak position is shifted to a larger angle side than the crank angle (i.e., an xcex1TDC) corresponding to the TDC. When the ignition ends unsuccessfully with a misfire, on the other hand, no pressure rise due to the explosion occurs so that the pressure assumes a maximum value at the TDC where the volume in the combustion chamber is at a minimum. After this, the piston moves down with the pressure change merely following a profile which is inverted relative to that of the rising time. Therefore, the internal pressure measured profile thus obtained is generally symmetric so as to have a peak position at the xcex1TDC. Thus, between the normal combustion time and the misfire time, differences apparently occur both in the peak value of the internal pressure measured profile and in the symmetry of the profile.
However, the misfire decision using only one point of the internal pressure peak value level has numerous error factors and lacks accuracy because it does not take into consideration the tendency of the entire profile. Japanese Patent Laid-Open Nos. 321752/1992, 72448/1992 and 325755/1992, therefore, discloses a method for deciding a misfire by comparing the magnitudes of a before top dead center pressure integrated value S1 obtained by integrating the internal pressure measured values for a constant integration period defined in the before top dead center period and an after top dead center pressure integrated value S2 obtained by integrating the average pressure information for a constant integration period defined in the after top dead center period. In short, by using the integrated values, the tendency of the entire internal pressure measured profile can be reflected in an average form on the decision result so that a more accurate misfire decision can be made.
Here, the method using the gasket type pressure sensor indirectly measures the internal pressure, in that the fastening force of the spark plug fixing the sensor is loosened by the internal pressure. As compared with the method of measuring the internal pressure directly through the pressure conduit by using the partition type sensor, therefore, there are a number of factors causing the absolute value of the measured pressure value to fluctuate. More specifically, variations in the force for fastening the spark plug and in the performance of individual piezoelectric elements easily cause the measured value level of the internal pressure to fluctuate, such that the measured value and its integrated value vary for a given internal pressure, easily resulting in an erroneous decision. In the above-specified patent publications, however, there is no disclosure of any specific means for reducing the adverse influences of these variation-causing factors.
It is therefore an object of the present invention to provide a misfire decision method and system for an internal combustion engine, which can be executed conveniently and inexpensively by using a gasket type pressure sensor and which can at all times make an accurate misfire decision while being hardly influenced by variations in the mounting arrangement or performance of the piezoelectric elements.
In order to achieve the above-specified object, the invention provides a misfire deciding method for an internal combustion engine, characterized by:
acquiring an internal pressure measured value based on the internal pressure of an internal combustion engine, to which a spark plug is attached, from the output of a pressure sensor mounted in a mounting seat of the spark plug;
calculating the integrated value of the internal pressure measured values for a first constant integration period in a period (as will be called the xe2x80x9cbefore top dead center periodxe2x80x9d) after an intake valve is closed and before a crank angle reaches top dead center, and setting the calculated value to a before top dead center integrated value S1;
calculating the integrated value of the internal pressure measured values for a second constant integration period in a period (as will be called the xe2x80x9cafter top dead center periodxe2x80x9d) after the crank angle reaches top dead center and before an exhaust valve is opened, and setting the calculated value to an after top dead center integrated value S2;
calculating a differential integrated value S1xe2x88x92S2 between the after top dead center integrated value S2 and the before top dead center integrated value S1; and
calculating a first correction reference value using the internal pressure measured value at a correction measurement point set for the before top dead center period, to correct the differential integrated value with the first correction reference value, to thereby make a misfire decision on the basis of the corrected differential integrated value.
The invention provides a misfire deciding system for an internal combustion engine, comprising:
a pressure sensor mounted in a mounting seat of a spark plug for acquiring an internal pressure measured value based on the internal pressure of an internal combustion engine having the spark plug mounted therein; and
a decision unit for: calculating the integrated value of the internal pressure measured values for a first constant integration period in a period (as will be called the xe2x80x9cbefore top dead center periodxe2x80x9d) after an intake valve is closed and before a crank angle reaches the top dead center, and setting the calculated value to a before top dead center integrated value S1; calculating the integrated value of the internal pressure measured values for a second constant integration period in a period (as will be called the xe2x80x9cafter top dead center periodxe2x80x9d) after the crank angle reaches top dead center and before an exhaust valve is opened, and setting the calculated value to an after top dead center integrated value S2; calculating a differential integrated value S1xe2x88x92S2 between the after top dead center integrated value S2 and the before top dead center integrated value S1; and calculating a first correction reference value using the internal pressure measured value at a correction measurement point set for the before top dead center period, to correct the differential integrated value with the first correction reference value, to thereby make a misfire decision on the basis of the corrected differential integrated value.
In the misfire deciding method and system of the invention, as shown in FIG. 4, the internal pressure measured profile for a constant period before and after the top dead center is determined so that the misfire decision may be made on the basis of the difference between the before top dead center pressure integrated value S1 and the after top dead center pressure integrated value S2, as obtained for the before top dead center period and for the after top dead center period, respectively, that is, the differential integrated value xcex94Sxe2x89xa1(S2xe2x88x92S1). The method of making the misfire decision on the basis of the differential integrated value xcex94S is convenient and is advantageous for easily canceling the influence of thermally caused drifts of the internal pressure measured values as described below. In either event, however, the misfire decision using the gasket type pressure sensor is easily caused to exert influences on the internal pressure measured value level due to variation in the fastening force of the sensor by the spark plug or in the performance of the piezoelectric element.
In the invention, therefore, the correction measurement point is set for the before top dead center period, and the first correction reference value is calculated using the internal pressure measured value at the correction measurement point, to correct the differential integrated value xcex94S with the first correction reference value, to thereby make the misfire decision on the basis of the corrected differential integrated value xcex94S. The correction measurement point set within the before top dead center period is relatively hardly influenced by the pressure rise due to the ignition/explosion of the fuel so that the internal pressure measured value obtained at the measurement point can be used as the reference value. By standardizing the individual measured values forming the internal pressure measured profile in comparison with that reference value, therefore, fluctuations in the internal pressure measured value levels due to the aforementioned factors can be suppressed to make a misfire decision of a higher accuracy.
At an idling time or at the time of running on level ground at constant speed, for example, the air-fuel ratio or the fuel consumption rate are substantially constant. If the crank angle at the correction measurement point is known, therefore, the internal pressure can be estimated from the volume of the combustion chamber at the correction measurement point. If the internal pressure measured value with the gasket type pressure sensor is corrected to match the estimated value of the internal pressure, therefore, the internal pressure measured value can be standardized even with variation in the sensor fastening force and in the piezoelectric element performance.
In the vigorously changing situation of the running state of the internal combustion engine, on the other hand, the absolute value of the internal pressure is difficult to estimate even if the correction measurement point is set at the common crank angle position. According to the following method, however, the correction to standardize the internal pressure measured value can be rationally made without such an absolute value estimation. As shown in FIG. 4, more specifically, two different correction measurement points are set within the before top dead center period, and a first correction reference value is calculated as a difference of xcex94P0xe2x89xa1P2xe2x88x92P1 between the internal pressure measured values P1 and P2 obtained for those two correction measurement points. If these two correction measurement points are individually set at constant crank angle positions, the volumetric change in the gas between the two correction measurement points always takes a determined value, and the difference xcex94P0 has a meaning as the reference pressure change corresponding to the constant volumetric change. The measured value of the difference xcex94P0 fluctuates with the aforementioned dispersions in the fastening forces or the piezoelectric element performances, and the internal pressure measured value to be used for the misfire decision also fluctuates with the same tendency. If the internal pressure measured value is displayed by correcting it with a relative value calculated by dividing it by the difference xcex94P0 for the reference pressure change, therefore, the influences of the aforementioned dispersions can be effectively reduced not through the absolute value correction of the internal pressure.
Next in the invention for making the misfire decision with the differential integrated value xcex94S, the value of (S1xe2x88x92S2)/xcex94P0 calculated by the division with the difference xcex94P0 of the internal pressure measured value or the first correction reference value is calculated (Formula (1) of FIG. 4) as a decision index xcex, and the misfire decision is made on the basis of the decision index xcex. Then, the misfire decision using the value xcex94S can be made more accurately and reproducibly. Even if the peak value of the internal pressure measured profile fluctuates by the aforementioned factors so that the measured value xcex94S itself takes a different value such as xcex94SA (=S2Axe2x88x92S1A) and xcex94SB (=S2Bxe2x88x92S1B), as shown in FIGS. 5(a) and 5(b), the influences of the fluctuations can be drastically reduced by dividing those values by the aforementioned differences xcex94POA and xcex94POB.
For these integrations, it is necessary to measure the internal pressure P as a function of time or crank angle. In FIG. 4, the internal pressure is measured as a function P(xcex1) of the crank angle xcex1. When the integration period of the before top dead center period is expressed by [xcex11, xcex1TDC] whereas the integration period of the after top dead center period is expressed by [xcex1TDC, xcex12], for example, the values S1 and S2 can be calculated on the basis of Formulas (2) and (3) of FIG. 4. In the computer processing, the values S1 and S2 are calculated by numerical integrations by sampling the internal pressure measured value P at every minute angles xcex4xcex1 while monitoring the crank angle xcex1 by a crank angle sensor or the like. In case the correspondence between the crank angle xcex1 and the time can be grasped, the integration variable should not be limited to the crank angle xcex1 but can be exemplified by the time t in a more convenient method. In order to lighten the influences of the engine speed (for a period of one cycle), it is then effective to average and use the time-integrated value of the internal pressure measured value P by the measured value of the prevailing engine speed.
Hereinafter, the integrated value of the internal pressure measured value P conceptionally includes not only the mathematically integral value (Formulas (2) and (3) of FIG. 4, as will be called the xe2x80x9cmathematically integrated valuexe2x80x9d) by the integral variable when the value P is expressed as a function of the crank angle xcex1 (or another parameter (e.g., the time t) which can correspond one to one to the value xcex1) as the integral variable but also another operation value, if this value reflects the integrated value. If the sampling interval of the internal pressure measured value is constant, for example, the added value of the sampled internal pressure measured values for a constant period is the operation value reflecting the mathematically integrated value so that it can be adopted as the integrated value, as defined herein. Moreover, the value calculated by dividing the mathematically integrated value or the added value by the width of the integration period or the number of added data expresses the average value of the internal pressure measured values for the individual periods and can be adopted as the integrated value, as defined herein.
If it is considered that the measurements of the internal pressure are ideally done, the differential integrated value xcex94S is zero at the misfire. It is, therefore, theoretically possible to decide the combustion to be a misfire, if the value xcex94S is zero, and to be normal if larger than zero. In the practical situations, however, due to various error factors (e.g., the influences due to the later-described hysteresis), the value xcex94S does not become zero but is measured as a finite value even at the misfire. In this case, more or less of a margin is introduced into the decision reference for the value xcex94S considering that error. In case the value xcex94S becomes smaller than a positive lower limit, it is effective for avoiding the erroneous decision to decide the misfire. Here in another method, the integration period of the before top dead center period is set so longer as to correspond to the aforementioned margin of the decision reference. Then, it is also possible to decide the misfire, if the value xcex94S takes zero or a negative value, and the normal combustion if larger than zero. In this case, strictly speaking, the integration period of the before top dead center period and the integration period of the after top dead center period are not equal.
In case the integration periods are set to an equal duration for the before top dead center period and the after top dead center period (that is, xcex1TDCxe2x88x92xcex11=xcex12xe2x88x92xcex1TDC: Formula (4) of FIG. 4), the following new effects can be attained. Specifically, the gasket type pressure sensor used for the internal pressure measurement has a sensor element comprising piezoelectric ceramics. On the other hand, most piezoelectric ceramics exhibit pyroelectrcity as the temperature rises. Therefore, the pressure sensor element using the piezoelectric ceramics has a problem that the zero point of the sensor output is liable to drift when the temperature changes. FIG. 6(a) schematically illustrates the internal pressure measured profile in a steady state (at a low temperature), and FIG. 6(b) schematically illustrates the internal pressure measured profile in a transient state (at a high temperature). By the influences of the zero-point drifts due to the temperature, the internal pressure measured profiles are evenly shifted over the entire measurement period, if the period is so short as to cause no problem in the temperature change. If the integration periods are equally set for the before top dead center period and the after top dead center period, therefore, the influences of the zero-point drifts can be offset at the operation of xcex94Sxe2x89xa1S2xe2x88x92S1, so that the misfire decision can be made more accurately.
In the misfire decision using the value xcex94S, for effectively retaining accuracy, the difference of the value xcex94S between the normal combustion time and the misfire time is as large as possible. From this viewpoint, it is effective that the ending point of the integration period set for the before top dead center period and the starting point of the integration period set for the after top dead center period are individually made identical to a top dead center xcex1TDC, as illustrated in FIG. 4. So long as the necessary and sufficient misfire decision accuracy can be retained when the integration period of the before top dead center period is set long for the aforementioned object, however, it is possible either to join the ending point of the integration period set for the before top dead center period and the starting point of the integration period set for the after top dead center period at a position (e.g., at a position deviated to the larger angle side) other than the top dead center xcex1TDC, or to provide a short non-integration period between the ending point of the integration period set for the before top dead center period and the starting point of the integration period set for the after top dead center period.
Next, for the aforementioned misfire decision, still another correction can be made by the following method. Specifically, a second correction reference value is calculated on the basis of the internal pressure measured value of the combustion cycle (or the estimated misfire cycle) estimated in advance to be the misfire cycle, to correct the differential integrated value xcex94S with the second correction reference value.
This correction is made effective by the following background intrinsic to the gasket type pressure sensor. FIG. 7(a) illustrates the results (in a solid curve) of the internal pressure measured value profile measured by the gasket type pressure sensor at the normal combustion time, in comparison with the results (in a broken curve) measured by a partition type standard pressure sensor through a pressure conduit formed in the cylinder head. It is thought that the measured values per se indicate the values more approximately from the true internal pressure. In the measurements by the gasket type pressure sensor, it is found that the profile at the pressure dropping time appears to shift to the higher pressure side than the profile at the pressure rising time. For example, FIG. 7(b) plots the measured value P of the seated pressure sensor corresponding to a common crank angle, against a measured value Pxe2x80x2 of a corresponding standard pressure sensor. It is found that the curves are different between the pressure rising time and the pressure dropping time thereby showing a clear hysteresis. On the other hand, FIG. 9 plots the decision index xcex obtained using two gasket type pressure sensors of common specifications, against a decision index xcex0 obtained by the standard sensor. By adopting the decision index xcex, the two linear curves have substantially equal gradients. It is, however, found that the values (i.e., xcexhA and xcexhB) at xcex0=0, i.e., at the misfire time indicate considerably different values due to the difference of the hysteresis.
This hysteresis is thought to occur because the compressive gas forced at the pressure rising time into the thread valley or gasket of the spark plug providing the portion to be mounted in the internal combustion engine is not promptly released at the pressure dropping time but remains. In either event, it is apparent from FIG. 7(a) that the profile at the pressure rising time is raised by the influences of the hysteresis to deteriorate the symmetry of the measured value profile curves important for the misfire decision. In the decision using the integrated values S1 and S2, the integrated value S2 of the after top dead center pressure is directly increased. It is, therefore, effective for improving the decision accuracy to correct the internal pressure measured value at the pressure dropping time thereby lessening the influences due to that hysteresis.
The frequency of occurrence of the hysteresis cannot be generally estimated unless the aforementioned standard sensor is used. Only at the misfire time, however, that frequency can be determined directly from the internal pressure measured value without providing the standard sensor. Specifically, the internal pressure measured value profile at the misfire time should theoretically be symmetric with respect to the top dead center position, as indicated by a broken curve in FIG. 8. However, the profile at the pressure dropping time is raised to the extent of the hysteresis, although the misfire occurs without the hysteresis. By comparing this profile with the profile at the rising time, therefore, it is possible to estimate the rising extent of the internal pressure measured value due to the hysteresis. In other words, the rising extent of the internal pressure measured value is calculated as the second correction reference value on the basis of the internal pressure measured value of the combustion cycle (or the estimated misfire cycle) which has been found in advance to become the misfire cycle.
In order to estimate the rising extent of the internal pressure measured value accurately, it is necessary to use the internal pressure measured value in the cycle which has been fixed for that at the misfire time, i.e., in an estimated misfire cycle. This internal pressure measured value ordinarily never fails to occur at the fuel cutting time for an abrupt deceleration while the internal combustion engine for an automobile is running. In case the misfire decision unit is commonly used for an ECU (Electronic Control Unit) for controlling the ignition timing or the air/fuel ratio of the internal combustion engine, for example, or in case the misfire decision unit can acquire its control information from the ECU although not used for the ECU, therefore, the misfire decision unit can grasp the occurrence of the estimated misfire cycle reliably and can calculate the second correction reference value without any problem.
As shown in FIG. 8, the second correction reference value can be calculated as a value reflecting the differential integrated value Sh in the estimated misfire cycle. In case the differential integrated value xcex94S itself is used as the misfire deciding information, the correction may be done by subtracting the value Sh or the second correction reference value from the value S2 of the value xcex94S. In the misfire decision using the aforementioned decision index xcex (=xcex94S/xcex94P0), on the other hand, the second correction reference value can use the decision index xcex obtained in the estimated misfire cycle, as a correction value xcexhp, and the decision index xcex obtained in the combustion cycle other than the estimated misfire cycle can be corrected, in a manner to subtract the correction value xcexhp. In either case, it is possible to make the accurate misfire decision, in which the influences of the hysteresis are lightened.
Here, the second correction reference value can be also calculated on the basis of the internal pressure measured value in a plurality of estimated misfire cycles having occurred in the past. For example, when the increment of the internal pressure measured value due to the hysteresis is expected to be varied with the time due to the occurring timing of the estimated misfire cycle, a more reliable value of the second correction reference value can be attained by further including a statistical process such as averaging the internal pressure measured values of a plurality of the estimated misfire cycles having occurred in the past.